The 724B is a TR cell, a device used to protect the receiver portion of a microwave transceiver which shares the same antenna for both transmit and receive functions. A TR cell functions like a triggered gap tube, and blocks the receiver hardware by shorting the receiver input during transmitter operation. Transmitter pulses cause the spark gap to break down, shorting the receiver input.

The 724B was used in several WWII-era 10GHz mobile radar systems, including the AN/APS-1, ASD (AN/APS-3) ASH (AN/APS-4) and AN/MPG-1.